1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) apparatus and image data generating method which enable generation of high-sensitivity image data by using an array coil.
2. Description of the Related Art
According to a magnetic resonance imaging (MRI) method, the spin of an atomic nucleus in a subject tissue placed in a static magnetic field is excited by using a radio-frequency signal (RF pulse) having a Larmor frequency associated with the atomic nucleus. Further, image data is reconstructed from a magnetic resonance (MR) signal generated with this excitation.
The MRI apparatus is a diagnostic imaging apparatus that utilizes the MRI method to generate image data of, e.g., the inside of a living body. Since the MRI apparatus can obtain many pieces of diagnostic information, e.g., not only anatomical diagnostic information but also biochemical information or diagnostic function information, this apparatus is important in the field of diagnostic imaging today.
In recent years, MRI using an array coil has been extensively carried out for the purpose of collecting image data with respect to an imaging region in a wide range with a high sensitivity. The array coil is constituted by two-dimensionally aligning a plurality of reception coils (which will be referred to as coil elements hereinafter). In MRI using the MRI array coil, MR signals detected by a plurality of coil elements adjacent to each other are added and combined with each other so that the plurality of coil elements are equivalently bundled in a plurality of sections. Furthermore, reconfiguring MR signals obtained from the respective sections enables generating a plurality of pieces of image data (referred to as original image data hereinafter). Moreover, combining the plurality of pieces of original image data enables the generation of image data used in diagnosis (referred to as diagnostic image data hereinafter) (see, e.g., JP-A 2003-175016 [KOKAI]).
FIG. 13 is a view showing an example of a positional relationship between a subject and an array coil 51.
The array coil 51 has three sections 52-1, 52-2, and 52-3. The array coil 51 is arranged in such a manner that an alignment direction of the sections 52-1 to 52-3 matches with a body axis direction of a subject.
FIG. 14 includes views showing three original images 53-1, 53-2, and 53-3 obtained by using the array coil 51. The original images 53-1 to 53-3 are obtained by respectively reconfiguring MR signals detected by coil elements belonging to the sections 52-1 to 52-3. The original image 53-1 has a maximum sensitivity in a region of the subject facing the coil element in the section 52-1. The original image 53-2 has a maximum sensitivity in a region of the subject facing the coil element in the section 52-2. Additionally, the original image 53-3 has a maximum sensitivity in a region of the subject facing the coil element in the section 52-3.
FIG. 15 is a view showing a diagnostic image 54 obtained by combining the original images 53-1 to 53-3 depicted in FIG. 14.
In MRI using the array coil, a coil element associated with an imaging region must be selected from a plurality of coil elements in the array coil arranged near a subject to form each of the above-explained sections. In particular, when imaging, e.g., a cervical spine or a spinal column, a diagnosis target region of the subject must be arranged above the array coil disposed on a top panel, and hence selecting a preferable coil element for an imaging region of this diagnosis target region is very difficult.
For example, when a coil element arranged outside the imaging region is erroneously selected, this coil element detects an MR signal having a high frequency component that does not satisfy a sampling theorem. In this case, an artifact due to aliasing in reconfiguration of the MR signal is produced in diagnostic image data. On the other hand, when a coil element arranged in the imaging region is not selected, a sensitivity in a region of diagnostic image data associated with a position of this coil element is considerably degraded.
Further, when an artifact or sensitivity degradation that cannot be allowed is recognized in diagnostic image data collected by using a first selected coil element, MRI must be again performed starting from selection of a coil element. When a skill of an operator is low, such an operation must be repeated until desired diagnostic image data is obtained. Therefore, not only an efficiency of MRI is considerably lowered but also a burden on the operator may be increased.